1. Field of the Invention
The present invention relates to a wind tunnel testing apparatus and methods for use in aerothermal testing, and more particularly to such materials and methods utilizing a low velocity subsonic flow with shear and heat flux conditions equivalent to a hypersonic or supersonic flight path of a test object.
2. Description of the Prior Art
The use of wind tunnels for simulating flight conditions is a common practice in the aerospace industry. Utilizing wind tunnels, objects are subjected to carefully controlled environmental conditions and monitored to determine how the object behaves. All wind tunnels operate under the same basic principle. A gas is accelerated to a desired velocity with a test object located within the gas stream. The conditions in the wind tunnel are controlled for a variety of parameters such as pressure, temperature, and velocity.
Wind tunnels are typically categorized depending on the speed of the gas flow that they produce. The gas speed is characterized by a Mach number, which is the speed of the gas divided by the speed of sound in the gas. Wind tunnels are generally divided into one of four categories depending on the Mach number of the gas flow: subsonic, transonic, supersonic, and hypersonic. Subsonic wind tunnels are the simplest of the four and are relatively inexpensive. They accelerate a gas to a speed less than that of sound, usually up to Mach 0.6. Increasing in complexity, but still relatively simple are transonic wind tunnels that provide for gas flows near the speed of sound, from Mach 0.6 up to Mach 0.9. Supersonic wind tunnels accelerate the gas beyond the speed of sound, over Mach 1, and hypersonic wind tunnels accelerate the gas to well beyond Mach 2. As the required gas velocity increases, the wind tunnel becomes increasingly more expensive to build and operate.
Traditionally, an object to be tested is subjected to environmental conditions that are similar to the environmental conditions the object will experience in normal operation. In most applications, this requires that the gas in the wind tunnel be heated and accelerated to a significant fraction of the flight speed of the test object. Additionally, to simulate high altitude conditions, the ambient pressure in the wind tunnel may need to be reduced.
Using present technology to test materials for a missile designed to fly at Mach 6 would require the test gas to be heated to 2500 F and the gas to be accelerated to 6 times the speed of sound. This has several drawbacks. First, in order to accelerate the gas to such a high velocity, a converging-diverging nozzle must be used. A converging-diverging nozzle is a nozzle that accelerates the flow of subsonic gas to the speed of sound as it converges at the throat, or minimum diameter of the nozzle, and then, as the nozzle diverges, the gas is further accelerated to supersonic and hypersonic speeds. Such nozzles are expensive to build and a separate nozzle is required for each Mach number of interest. The nozzle must be as large as, if not larger, than the object to be tested. In addition to being expensive to build, massive amounts of gas must be available to supply the high mass flow rates encountered with such a large nozzle. Finally, the nozzles themselves must be kept cool, thus requiring large cooling systems. These limitations restrict the practical size of the nozzle commonly resulting in use of a nozzle that is undersized for a given application.
Aerothermal testing examines how a material responds to conditions of high temperatures and viscous shear forces. In applications that are not shear sensitive, matching the heat flux to that expected under operational flight conditions is sufficient. Testing other materials, such as ablators, requires that both the heat flux and shear force must be simultaneously matched. The parameters that primarily contribute to the heat flux and shear force are the pressure, temperature, and Mach number of the gas to which the test object is subjected. For any given point on a high-speed flight trajectory, an equivalent subsonic flow condition having a different pressure and temperature exists that will provide matching heat flux and shear conditions. The required values of pressure, temperature, and Mach number are calculated using conventional methods such as standard closed-form empirical boundary layer approximations, computational fluid dynamics, or other computational programs such as ATAC available from ITT Aerotherm.
Prior attempts at simulating high-speed aerothermal conditions with a subsonic test section relied on a diffuser from which the test gas exited directly into atmospheric pressure or into a controlled pressure environment. With this arrangement, it was not possible to control the test section pressure or Mach number and hence it was only possible to simulate the heat flux over the test article. Other problems were encountered due to a mismatch between the required mass flow and the scale of the diffuser and resulted in a non-uniform flow condition over the test article.